Chronic administration of ethanol produces dependence and tolerance. The induction of ethanol dependence in rats is associated with an alteration in the viscosity of the synaptosomal plasma membrane (SPM). There is also a marked increase in the amount of cholesterol in the SPM of animals dependent on and tolerant to ethanol. Two phospholipid methylating enzymes are localized in the SPM. The activity of these two enzymes is decreased in SPM of animals dependent on ethanol.